Intermicrotubule Bridges in Mitotic Spindle Apparatus

Fine interconnections or "cross-bridges" have been observed between cytoplasmic microtubules in many different organisms (1, 3, 7, 9, 10, 12, 13) . Recently, similar bridges have been noted between the microtubules of the spindle apparatus in dividing plant and animal cells (4, 6, 7, 11, 14) . Increasing attention is being directed toward intertubule bridges, since they may play an important role in microtubule-mediated motility (1, 8, 9) . In this brief note we present our observations on the structure of arms and bridges on the microtubules of the spindle apparatus in cultured human cells (HeLa and WI-38) and in the endosperm of the African blood lily, Haemanthus katherinae . We demonstrate the presence of bridges between kinetochore tubules, between continuous tubules, and, during later stages of mitosis, between the tubules of the interzone . We also show bridges between tubules and vesicles in the postmitotic stem of HeLa cells and in the phragmoplast of Haemanthus .